1. Technical Field
The present invention relates to an electrolyte, which has improved safety and stability, for use in high voltage batteries, and to a lithium secondary battery comprising the electrolyte. Particularly, the present invention relates to an electrolyte that can operate safely when charged at a charging voltage of up to 4.35V or higher, and to a lithium secondary battery comprising the electrolyte.
2. Background Art
As the use of portable devices, such as high-performance notebook computers and mobile phones becomes increasingly popular all over the world, the demand for high-performance secondary batteries with high energy density is increasing exponentially. Particularly, lithium ion secondary batteries are being increasingly applied in most portable electronic products despite their relatively recent introduction to the market, and thus, studies to extend the run-time of portable devices by increasing the capacity of lithium secondary batteries are being actively conducted. In addition, for portable electronic products using display devices such as liquid crystal displays (“LCDs”), a more powerful lithium ion battery is both increasingly necessary and desirable for more efficient performance of such devices. However, since increases in the battery capacity and in the operating voltage of batteries can lead to deterioration in the battery safety, various attempts to improve the safety of lithium secondary batteries have been made.
Putting together the results of studies conducted up to now on the safety of the lithium secondary batteries, the thermal stability of active material and electrolyte in a charged state has the greatest effect on the battery safety. For this reason, studies to improve the safety of the lithium secondary batteries consist mainly of attempts to develop positive active materials with excellent thermal stability and studies to improve the thermal stability of electrolytes.
At present, the solvents used in electrolytes for the lithium secondary batteries are mainly cyclic and linear carbonates. Such solvents are inflammable and thus, upon either an increase in temperature as caused by local short circuits within the batteries, or an increase in surrounding temperature, the solvents will easily react with oxygen generated by the structural degradation of an active material in the battery, particularly a positive active material, resulting in combustion and explosion. For this reason, imparting flame retardance to the electrolytes will greatly contribute to the improvement of the battery safety.
Many studies on flame retardant electrolytes which can be used in the lithium secondary batteries have been conducted, centering on either solvents containing a compound having a fluorine-for-hydrogen substitution in carbonate or solvents containing phosphorus. Such a solvent has lower flammability and combustibility than those of the prior carbonate or ester solvents, but needs to be used in large amounts in order that the electrolyte has sufficient flame retardance. Also, such flame retardant solvents show a lower solubility toward lithium salts than the existing cyclic carbonates and contain fluorine or phosphorus having a higher atomic weight than that of hydrogen in the unsubstituted cyclic carbonates, resulting in a great increase in the viscosity of electrolytes. Thus, if the volume ratio of this flame retardant solvent in electrolyte solvent increases, the performance of batteries will greatly deteriorate due to a reduction in lithium ion conductivity.
Japanese Patent Laid-open Publication No. Hei 10-199567 discloses that if trifluoropropylene carbonate (i.e., trifluoromethyl-substituted ethylene carbonate, “TFPC”) of the following formula 1 is used at the amount of 60-90% by volume relative to the total volume of electrolyte solvent, the safety of batteries can be improved:

However, trifluoropropylene carbonate has about two times higher viscosity than that of ethylene carbonate or propylene carbonate, each a generally used solvent. Thus, if trifluoropropylene carbonate is used at the amount described in the Japanese publication, it will result in a significant reduction in the ion conductivity of the electrolyte formed therewith, thus making the deterioration of battery performance inevitable. Moreover, as the trifluoropropylene carbonate is a propylene carbonate substituted with fluorine, it has some of the general disadvantages of propylene carbonate. In particular, if it is used in an electrolyte solvent, the stability of any coating layer formed at the interface between a graphite negative electrode and an electrolyte will be adversely affected and therefore insufficient to a degree, and problems in the charge/discharge life cycles of the battery will occur, as described in Electrochimica Acta Vol. 45, p. 99, (1999).
U.S. Pat. No. 6,506,524 describes a solvent consisting of fluoroethylene carbonate and propylene carbonate used as electrolyte solvent and the resulting electrolyte-stable protective layer capable of being formed on the surface of a graphite negative electrode material. However, if the solvent of this composition is used as electrolyte solvent, the ion conductivity of electrolyte will be reduced to less than 7 mS/cm, thus deteriorating the performance of batteries, because fluoroethylene carbonate and propylene carbonate have high dielectric constant but undesirably high viscosity.
When the ion conductivity of electrolyte is reduced to less than 7 mS/cm, the conducting properties of the ions in the electrolyte deteriorate and the movement of the lithium ions becomes slow, resulting in an overall deterioration of the battery performance.